This invention is directed to a line pressure control system for a continuously variable transmission (CVT). Such a transmission includes fluid-actuated primary and secondary pulleys intercoupled by a flexible belt, the effective diameters of the pulleys being varied simultaneously but in opposite directions to effect a smooth, continuous change in the drive ratio between an input driving shaft and an output driven shaft. The first (primary) pulley is usually controlled by fluid under pressure to regulate the effective drive ratio of the transmission, while the secondary pulley is supplied with fluid at a pressure sufficient to prevent slippage of the belt in order to transfer torque from the primary pulley to the secondary pulley. A fluid-actuated clutch is generally employed to transfer drive torque from the transmission (namely, from the secondary pulley) to an associated driveline. The line pressure is the highest pressure in the CVT and is that which is applied to the secondary pulley to ensure an adequate clamping force and tension on the belt so that it will not slip.
Various systems have been developed for controlling the different hydraulic pressures required to operate a CVT, such as those systems described in copending U.S. patent applications Ser. Nos. 717,913 and 722,578, filed on Mar. 29, 1985 and Apr. 12, 1985, now U.S. Pat. Nos. 4,718,308 and 4,648,496 respectively, and assigned to the present assignee, and also in the various references mentioned in the copending applications. The present invention constitutes an improvement over those prior control systems in that more reliable operation is achieved, while at the same time employing a simpler and less expensive arrangement than that required in the prior systems. For example, in order to realize closed loop control of both the line (secondary) pressure and the clutch pressure in the past, two separate pressure transducers or sensors would be needed. This is accomplished in the present invention by means of only one such transducer.
As another example, if a closed loop line pressure controller does not respond fast enough to changes in a desired setpoint, the line pressure is likely to become less than the level desired and the belt could slip, causing damage to itself and the sheaves. This cannot occur in the present invention. In the event that the closed loop controller cannot keep up with the setpoint changes, an open loop controller takes over and ensures that the line pressure will always be greater than the level called for by the setpoint.
As a further example, this open loop controller makes use of a look-up table to control the line pressure. Typically this would require a large amount of stored data. In the case of the present invention, the amount of memory required is significantly reduced by the use of an interpolation algorithm.